Oh man, would you mind talking a little more about your pet HC of ace!Dean? I have flirted with aro!Dean for ages, but now that you mention ace...!
This is intended to be a fic one day so I've sort of avoided talking about it, but tbh it might be one of those headcanons that doesn't really deserve a fic? Like the kind of thing where, once it's translated into narrative, it really just reads like a pamphlet for the author's HC and isn't particularly satisfying to read. So I guess I might as well talk about it.
First: **THIS IS JUST A HEADCANON OMG**. Here I'm using headcanon in the sense of 'hey, this is a thing that can sorta be extrapolated from the text as shown, and it's kinda fun to play with! I don't think it's literally canon or intentional on the part of the writers, but it's entertaining!' So, please don't take as me trying to claim canon gospel, I don't need angry anons. Especially these days.
Second, to the aro thing: I know a lot of people do kinda like that for Dean, and to be honest I can't see it at all, like at all at all. But! There are a lot of semantics and personal interpretations when we come to the words 'love' or 'romance', so what you may viewing as a divorce between Dean and either concept may be miles from what I'm viewing as Dean + those concepts. I see a much more fluid/seamless river between affection/friendlove/romanticlove/need-for-closeness. This is aside from the fact that Dean's love for Sam is pretty indefinable and insane-making, in literal canon as well as any fanon treatments.
That said, I really do see a lot of romantic urges in Dean. Of course they can be interpretable depending on your semantic readings! But things like: - crushing on the guitar lesson girl in Bad Boys - immediately falling in love with Cassie - the dream of himself in the djinn!episode being in a committed affectionate dating relationship with Carmen - literally dreaming about Hallmark picnic dates with Lisa - later actually committing to a real relationship with Lisa, and fully operating as a romantic partner for her. His speech in the virginity ep about how good sex is because of connection, and his wistfulness when he talks about how he always has to say goodbye. [from here on I'm going to stop putting in caveats about how those *can* be interpreted still as aro and just take it as read that you can have any hc you like, okay?]
Third, given the last item in the previous list: I find it fun to interrogate Dean's love for sex. He loves sex. He loves it! He goes after it when he can get it and has so much fun! There's the fireworks and the joy and all that good stuff. ...But. Physical sensations are nice, but what if it's more that he wants the warm and happy human connection? What if it's that he loves making other people feel good, because after all what is Dean but the happiest-to-serve little dumpling? What if, after all, he doesn't necessarily need sex in itself just to get off, but instead really is after the romantic elements of the act, however he can access them?
Is that 100% supported by canon? Of course not. He's going after purple nurple girl because he wants to get laid. I just find it to be an interesting read, especially as we see his 'hookup count' go down and down through the years, until he's practically celibate in the latest part of the show when he's living in the bunker with Sam in what is, romantically or not, a committed relationship. Dean doesn't fuck prostitutes and he does not coldly get laid -- there was NO way he was going to have hate!sex with Bela -- so the sex qua sex is not the goal. He wants affection and connection, not just an orgasm.
As often is the case, the reason I'm interested in something is because it's a surprisingly available read in the text, and I think this sort of is! Of course he would have to be a sex-positive ace person, not of the repulsed type. But what I see, for the fic that will probably never get written, is a wincest life where he and Sam started hooking up whenever, that part doesn't matter -- but now they're in the bunker and they're committed and they're never letting go, and Sam realizes that Dean hasn't initiated sex in... months. They have had sex, of course! But Sam's realizing that he's always the one who starts it -- and Dean happily leans in and goes along and everyone seems to have a good time. But what's really happening is that Dean's getting the warm intimate emotional connection that he needs; orgasms really are just an afterthought.
I find it interesting, that's all. Mr. soft saxophone solo, holding hands during sex, is really more interested in the holding hands. Sam should take him for a picnic.